Chao Luo

Change in atmospheric mineral aerosols in response to climate: Last glacial period, preindustrial, modern, and doubled carbon dioxide climates

Desert dust simulations generated by the National Center for Atmospheric Researchs Community Climate System Model for the current climate are shown to be consistent with present day satellite and deposition data. The response of the dust cycle to last glacial maximum, preindustrial, modern, and doubled-carbon dioxide climates is analyzed. Only natural (non-land use related) dust sources are included in this simulation. Similar to some previous studies, dust production mainly responds to changes in the source areas from vegetation changes, not from winds or soil moisture changes alone. This model simulates a +92%, +33%, and −60% change in dust loading for the last glacial maximum, preindustrial, and doubled-carbon dioxide climate, respectively, when impacts of carbon dioxide fertilization on vegetation are included in the model. Terrestrial sediment records from the last glacial maximum compiled here indicate a large underestimate of deposition in continental regions, probably due to the lack of simulation of glaciogenic dust sources. In order to include the glaciogenic dust sources as a first approximation, we designate the location of these sources, and infer the size of the sources using an inversion method that best matches the available data. The inclusion of these inferred glaciogenic dust sources increases our dust flux in the last glacial maximum from 2.1 to 3.3 times current deposition.

Global distribution of atmospheric phosphorus sources, concentrations and deposition rates, and anthropogenic impacts

A worldwide compilation of atmospheric total phosphorus (TP) and phosphate (PO4) concentration and deposition flux observations are combined with transport model simulations to derive the global distribution of concentrations and deposition fluxes of TP and PO4. Our results suggest that mineral aerosols are the dominant source of TP on a global scale (82%), with primary biogenic particles (12%) and combustion sources (5%) important in nondusty regions. Globally averaged anthropogenic inputs are estimated to be similar to 5 and 15% for TP and PO4, respectively, and may contribute as much as 50% to the deposition over the oligotrophic ocean where productivity may be phosphorus-limited. There is a net loss of TP from many (but not all) land ecosystems and a net gain of TP by the oceans (560 Gg P a(-1)). More measurements of atmospheric TP and PO4 will assist in reducing uncertainties in our understanding of the role that atmospheric phosphorus may play in global biogeochemistry.

Is ozone pollution affecting crop yields in China

Newly available data from non-urban locations in China along with regional model simulations suggest that ground-level ozone may be sufficiently high to affect Chinas winter wheat production. As non-urban ozone increases with industrialization, its effects on crops could hinder efforts to meet increasing food demands in the coming decades, in China.

Understanding the 30‐year Barbados desert dust record

Atmospheric mineral aerosols influence climate and biogeochemistry, and thus understanding the impact of humans on mineral aerosols is important. Our longest continuous record of in situ atmospheric desert dust measurements comes from Barbados, which shows fluctuations of a factor of 4 in surface mass concentrations between the 1960s and the 1980s [Prospero and Nees, 1986]. Understanding fluctuations this large should help us understand how natural and anthropogenic factors change mineral aerosol sources, transport, distributions, and deposition, although we are limited in our ability to interpret the results as there is a quantitative record only at one location. We test the hypothesis that dry topographic lows (and not disturbed sources such as cultivated areas or new desert regions) are the sources of desert dust, using a hierarchy of models as well meteorological data sets to look at decadal scale changes in the North Atlantic desert dust. We find that the inclusion of a disturbed source improves our simulations in many (but not all) comparisons. Unfortunately, we are severely limited by the accuracy of the available data sets and models in making definitive statements about the role of disturbed sources or anthropogenic activity in changing the atmospheric desert dust cycle. Processes that might change the size or intensity of desert dust sources in North Africa (such as new sources due to desertification or land use) may be difficult to distinguish from topographic low sources in models due to their similar geographical locations and impact on atmospheric aerosol distributions.

Impacts of atmospheric nutrient inputs on marine biogeochemistry

The primary nutrients that limit marine phytoplankton growth rates include nitrogen (N), phosphorus (P), iron (Fe), and silicon (Si). Atmospheric transport and deposition provides a source for each of these nutrients to the oceans. We utilize an ocean biogeochemical model to examine the relative importance of these atmospheric inputs for ocean biogeochemistry and export production. In the current era, simulations with the biogeochemical elemental cycling ocean model suggest that globally, atmospheric Fe inputs could support roughly 50% of the Fe exported from the euphotic zone by sinking organic and inorganic particles. Variations in atmospheric iron inputs strongly impact spatial patterns of phytoplankton growth limitation and the areal extent of the high-nutrient, low-chlorophyll regions. Atmospheric inputs of N, Si, and P have smaller impacts, potentially accounting for 5.1%, 0.21%, and 0.12% of the biogenic export of these elements from the euphotic zone, respectively. Soluble Fe input from the atmosphere is sufficient to support most of the export production in many ocean regions, whether we use a spatially variable aerosol Fe solubility, or a globally constant 2% solubility. Regionally atmospheric N inputs can have significant impacts on marine biogeochemistry, potentially supporting >25% of the export production, an impact that is increasing due to human activities. Atmospheric Si and P inputs have only minimal impacts on marine ecosystem productivity and biogeochemistry, as these inputs are typically quite small relative to the flux of these nutrients from below the euphotic zone.

Impacts of increasing anthropogenic soluble iron and nitrogen deposition on ocean biogeochemistry

[1] We present results from transient sensitivity studies with the Biogeochemical Elemental Cycling (BEC) ocean model to increasing anthropogenic atmospheric inorganic nitrogen (N) and soluble iron (Fe) deposition over the industrial era. Elevated N deposition results from fossil fuel combustion and agriculture, and elevated soluble Fe deposition results from increased atmospheric processing in the presence of anthropogenic pollutants and soluble Fe from combustion sources. Simulations with increasing Fe and increasing Fe and N inputs raised simulated marine nitrogen fixation, with the majority of the increase in the subtropical North and South Pacific, and raised primary production and export in the high-nutrient low-chlorophyll (HNLC) regions. Increasing N inputs alone elevated small phytoplankton and diatom production, resulting in increased phosphorus (P) and Fe limitation for diazotrophs, hence reducing nitrogen fixation (6%). Globally, the simulated primary production, sinking particulate organic carbon (POC) export. and atmospheric CO2 uptake were highest under combined increase in Fe and N inputs compared to preindustrial control. Our results suggest that increasing combustion iron sources and aerosol Fe solubility along with atmospheric anthropogenic nitrogen deposition are perturbing marine biogeochemical cycling and could partially explain the observed trend toward increased P limitation at station ALOHA in the subtropical North Pacific. Excess inorganic nitrogen ([NO3 ] + [NH4 ] 16[PO4 ]) distributions may offer useful insights for understanding changing ocean circulation and biogeochemistry.

Effects of atmospheric inorganic nitrogen deposition on ocean biogeochemistry

We perform a sensitivity study with the Biogeochemical Elemental Cycling (BEC) ocean model to understand the impact of atmospheric inorganic nitrogen deposition on marine biogeochemistry and air-sea CO2 exchange. Simulations involved examining the response to three different atmospheric inorganic nitrogen deposition scenarios namely, Pre-industrial (22 Tg N/year), 1990s (39 Tg N/year), and an Intergovernmental Panel on Climate Change (IPCC) prediction for 2100, IPCC-A1FI (69 Tg N/year). Globally, the increasing N deposition had widespread, but modest effects on export production and air-sea CO2 exchange. The maximum increase in N deposition was 47 Tg N/year since Pre-industrial control for the IPCC-A1FI case, which had an increase in primary production (0.98 Gt C/year or 2%), export production (0.16 Gt C/year or 3%) and a decrease in atmospheric pCO2 of 1.66 ppm (0.6%) relative to the Pre-industrial control. In some regions, atmospheric N inputs supported >20% of the export production in the current era and >50% of the export production in the IPCC-A1FI case. As nitrogen deposition increased, N fixation decreased because the diazotrophs were outcompeted by diatoms and small phytoplankton under more N-replete conditions. This decrease in N fixation could partially counteract the ongoing increase in new nitrogen inputs via atmospheric N deposition.

Regional simulation of anthropogenic sulfur over East Asia and its sensitivity to model parameters

We discuss a series of simulations of anthropogenic sulfur over East Asia with a SO2/SO42− chemistry-transport model driven in on-line mode by a regional climate model. Sensitivity to OH and H2O2 concentration, cloud parameters, SO2 dry deposition and emission strength is analyzed and the different components of the sulfur budget are examined. The SO2 and SO2−4 column burdens show pronounced variability at temporal scales from seasonal to synoptic and sub-daily, with SO2 and SO2−4 behaving differently due to the interplay of chemical conversion, removal and transport processes. Both SO2 and SO2−4 show marked spatial variability, with emission being the dominant term in regulating the SO2 spatial distribution. The atmospheric SO2 and SO2−4 amounts show close to a linear response to surface emission. Aqueous phase SO2→SO2−4 conversion and wet removal are the primary factors that regulate the SO2−4 amounts, with dry deposition and gas phase SO2→SO2−4 conversion being of secondary importance. Aqueous phase conversion and dry deposition are the dominant loss mechanisms for SO2 . The model shows low sensitivity to variations in OH, H2O2, and cloud parameters, while the sensitivity to prescribed dry deposition velocity is more pronounced. Overall, our results are in line with previous modeling studies and with very limited available observations.

The Role of Easterly Waves on African Desert Dust Transport

Abstract Mineral aerosols from North Africa represent one of the largest sources of aerosols available to the atmosphere, and their generation and transport are thought to be modulated by African easterly waves. In this study, the relationships between easterly wave activity and model simulations of desert dust entrainment and transport are investigated. National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis datasets are used to both evaluate easterly wave activity and drive a transport model simulation of desert dust. The focus of this study is on boreal summer, when easterly wave activity maximizes. Periods of high easterly wave activity are identified using filtered (2.5–10 days) relative vorticity at 700 hPa over the tropical Atlantic Ocean. Lag composites of relative vorticity and simulated surface dust concentrations are used to investigate the influence of easterly waves on the spatial transport patterns. A comparison between lag composites of available in...

Response of a coupled chemistry‐climate model to changes in aerosol emissions: Global impact on the hydrological cycle and the tropospheric burdens of OH, ozone, and NOx

[1] In this study, we analyze the response of the coupled chemistry-climate system to changes in aerosol emissions in fully coupled atmospheric chemistry-climate-slab ocean model simulations; only the direct radiative effect of aerosols and their uptake of chemical species are considered in this study. We show that, at the global scale, a decrease in emissions of the considered aerosols (or their precursors) produces a warmer and moister climate. In addition, the tropospheric burdens of OH and ozone increase when aerosol emissions are decreased. The ozone response is a combination of the impact of reduced heterogeneous uptake of N2O5 and increased ozone loss in a moister atmosphere. Under reduced aerosol emissions, the tropospheric burden of NOx (NO + NO2) is strongly reduced by an increase in nitric acid formation but also increased by the reduced N2O5 uptake. Finally, we discuss the significant difference found between the combined impact of all aerosols emissions and the sum of their individual contributions. Citation: Lamarque, J.-F., J. T. Kiehl, P. G. Hess, W. D. Collins, L. K. Emmons, P. Ginoux, C. Luo, and X. X. Tie (2005), Response of a coupled chemistryclimate model to changes in aerosol emissions: Global impact on the hydrological cycle and the tropospheric burdens of OH, ozone, and NOx, Geophys. Res. Lett., 32, L16809, doi:10.1029/ 2005GL023419.